Diagnosis: Locomotive form usually between 12 and 40 µm in length. Pseudopodia clear, slowly eruptive, and occasionally branching at base. With numerous, slender, and tapering subpseudopodia (acanthopodia) giving the cell a spiny appearance. Cytoplasm without crystals but frequently with small lipid globules and an obvious contractile vacuole. Nucleus spherical with a central nucleolus. Posterior uroid temporary. Cysts common, often with 2 layers, a thick, wrinkled outer ectocyst and an inner polygonal endocyst and a wall pore with operculum.Type species: Acanthamoeba castellanii (Douglas, 1930).

Ecology: Possibly the most commonly isolated genus of all naked amoebae from freshwater and soil habitats. Acanthamoebae have also been isolated from salt water of low salinity, presumably from cysts.

Remarks: Some Acanthamoeba have been implicated in infections of the human cornea (Acanthamoeba Keratitis) and include: A.castellani, A. polyphaga, A. hatchetii, A. culbertsoni, A.rhysodes and A. griffin.
At least 35 named species which can be distinguished, with difficulty, on the basis of their cyst morphology, immunofluorescence and on isoenzyme structure. Non-morphological procedures (i.e. chemotaxonomic approaches) have been applied to the genus and readers should consult Byers et al. (1983), Costas and Griffiths (1985), McLaughlin et al. (1988), Johnson et al. (1990).

Diagnosis: During locomotion amoebae are flat in cross section and vary from nearly circular in outline to flabellate to elongate to sometimes branching. Locomoting amoebae are typically longer than they are wide along the axis of motility, but may occasionally be wider than long. Mean cell length is 26.9 μm (standard deviation = 4.2 μm, n = 30) and mean cell breath is 19.3 μm (standard deviation = 3.8 μm, n = 30). The leading edge of the locomoting amoeba is a lobose, hyaloplasmic pseudopodium that typically supports acanthapodia. The pseudopodium usually makes up 20-25% of the length of the amoeba. Acanthopodia may extend from all around the circumference of the cell. There is typically no uroid. The granular cytoplasm contains a single, spherical to subspherical nucleus (mean diameter is 5.1 μm) with a central to slightly eccentric nucleolus (mean diameter is 2.3 μm) that is never more than half the diameter of the nucleus, and often less. There is usually a single contractile vacuole that is typically located posterior to the nucleus in locomoting cells within a distance of one nuclear diameter. At diastole, the contractile vacuole is equal to or greater in diameter than the nucleus. When cells round up during mitosis (not illustrated), they become circular in outline with short acanthapodia radiating from their entire circumference. These acanthapodia are present from prophase through early cytokinesis. Cysts are mostly isodiametric with stellate knobs, with a mean diameter of 13.1 μm (n = 3). The cyst walls appear to have only one wall layer when viewed with light microscopy. Sporocarps develop from a prespore cell that develops as an amoeba rounds up and becomes refractile. Just before stalk deposition begins the prespore cell assumes an ellipsoid shape then becomes nearly spherical. The prespore transitions to the sporogen stage as stalk deposition begins. The sporogen is obpyriform, and the stalk is deposited in an invagination of the narrowed lower portion of the sporogen. At maturity, the sporogen lays down a spore wall and becomes an obpyriform spore with an invagination into which the apex of the stalk is inserted. The spore is deciduous and is easily removed from the stalk by air currents. Before the spore is shed, it waves around, flags, readily in air currents. The stalk is several times the diameter of the spore in length and tapers upward from a distinct basal disk to a narrow column. When the spore is shed, the apex of the stalk, which was inserted into the invagination of the spore, can be seen to swell into a knob-like swelling, the apophysis, that appear to be solid when viewed with light microscopy.